Analog switch circuit

ABSTRACT

An analog switch and an analog multiplexer are realized by which electron charges which have been stored in a stray capacitance provided on the output side thereof before a switch is conducted do not give an adverse influence to a level of such an analog input voltage which is subsequently entered after the switch has been switched. An analog switch circuit is arranged by insulating gate type transistors and a voltage follower which is parallel-connected to these insulating type transistors. When the analog switch circuit is turned ON, the voltage follower is firstly brought into an active state, and thereafter, these insulating gate type transistors are brought into conductive conditions.

BACKGROUND OF THE INVENTION

[0001] The present invention is related to an analog switch, and asample-and-hold circuit for an analog input voltage. More specifically,the present invention is directed to such a technique capable ofreducing an adverse influence caused by such a voltage which has alreadybeen inputted into an analog multiplexer. For instance, the presentinvention is pertinent to such an effective technique which is utilizedin an A/D converting circuit, and also a single-chip microprocessorcomputer having the A/D converting cirvcuit therein.

[0002] In an A/D (analog-to-digital) converting circuit, and asemiconductor integrated circuit containing this A/D converting circuit,an analog switch, and a sample-and-hold circuit are employed. The analogswitch is used so as to conduct an analog input voltage from an externalsource, and this analog input voltage should be A/D-converted. Thesample-and-hold circuit holds the analog input voltage while beingA/D-converted. Also, in the case that there are plural analog signalsderived from a plurality of sensors as realized in a control system foran automobile, when each of these plural analog signals is A/D-convertedinto a digital signal by using a respective one of the individual A/Dconverting circuits, a total number of these A/D converting circuitsbecomes large, so that cost of the automobile control system would beincreased. As a consequence, such an automobile control system may besometimes arranged in such a manner that a plurality of analog inputvoltages are A/D-converted in a time divisional manner by employing asingle A/D converting circuit.

[0003]FIG. 20 represents such a time-divisional type A/D convertingsystem. This system is arranged as follows: That is, in this A/Dconverting system, a plurality of analog input voltages Vin1,Vin2, - - - , Vinn are selected by a multiplexer MPX one by one, andthen, the selected analog input voltages are acquired by asample-and-hold circuit SH. Thus, a plurality of analog input voltagesare A/D-converted in the time divisional manner by using a single A/Dconverting circuit ADC.

SUMMARY OF THE INVENTION

[0004] In the A/D converting system as indicated in FIG. 20, analogswitches SWc1 to SWcn which constitute the multiplexer MPX, and asampling switch SWs of the sample-and-hold circuit SH are constituted byemploying MOSFETs. On the other hand, in the A/D converting system ofFIG. 20, a stray capacitance Ca is produced at a connection node NObetween the analog switches SWc1 to SWcn of the respective channelswhich constitute the multiplexer MPX, and the sampling switch SWs whichconstitutes the sample-and-hold circuit SH.

[0005] The production of the above-described stray capacitance Ca iscaused by a junction capacitance and a wiring capacitance between asource (drain) and a main body (well) of an MOSFET. When the Inventorsof the present invention tried to calculate such a stray capacitance ofa circuit having 8 channels, this stray capacitance was approximately 30pF (picofarad). Also, another stray capacitance Ca is produced at aninput terminal of an A/D converting circuit 30 of FIG. 20. Furthermore,as a capacitance value of a sampling capacitor Cs which constitutes thissample-and-hold circuit SH, such sampling capacitors having capacitancevalues of, for example, approximately 5 to 6 pF are used.

[0006] On the other hand, in order to improve the responsecharacteristic of the sample-and-hold circuit SH, impedances(ON-resistances) of the analog switches SWc1 to SWcn, and also animpedance (ON-resistance) of the sampling switch SWs are preferably madelow. For instance, in the case that sampling time is selected to be onthe order of 3 μs (microseconds), an impedance of such a sample-and-holdcircuit may be preferably selected to be lower than, or equal to 1 kΩ(killoohms).

[0007] However, the following risk problem can be revealed. That is,when the channels are switched by the multiplexer MPX, if the impedancesof the analog switches SWc1 to SWcn are low and further the straycapacitance Ca is produced at the connection node NO between the analogswitches SWc1 to SWcn and the sampling switch SWs, the electron chargewhich has been stored in this stray capacitance Ca before the channelsare switched may give an adverse influence to a level of a next analoginput voltage which is inputted via such an analog switch which isturned ON after the channels are switched by the multiplexer MPX. As aresult, an error of the analog input voltage to be sampled would beincreased.

[0008] To avoid this problem, in such a control system as shown in FIG.20 in which while the channels are switched by the multiplexer MPX, theA/D converting operation is carried out in the time divisional manner,it is preferable to connect externally-connectable capacitors Ci1 to Cineach having capacitances of approximately 0.1 μF to the respectiveanalog input terminals AIN1 to AINn. The reason why the error can bereduced is given as follows. That is, when such externally-connectablecapacitances Ci1 to Cin are connected to these analog input terminalsAIN1 to AINn, electron charges are redistributed via such an analogswitch which is turned ON between the stray capacitance Ca and any oneof these externally-connectable capacitors Ci1 to Cin, which may reducethe error of the analog input voltage to be sampled.

[0009]FIG. 21 graphically shows a relationship between an internalimpedance Rin of an analog signal source and an error δ (LSB) of ananalog input voltage Vin to be sampled, assuming now that thecapacitance values of the externally-connectable capacitors Ci1 to Cinwhich are connected to the analog input terminals AIN1 to AINn areconstant. In this case, it is so assumed that the error δ (LSB) may beexpressed by the following formula (1) under such a condition thatresolution of the A/D converting circuit is selected to be 10 bits, areference voltage is selected to be Vref, and also a voltage which isactually acquired into the sampling capacitor Cs is selected to be “Vs”:

δ (LSB)=(Vin−Vs)/(Vref/1024)   (1)

[0010] In this drawing of FIG. 21, a solid line “a” indicates such anerror plotted in the case that the capacitance value of theexternally-connectable capacitor Ci is equal to 0.1 μF. A broken line“b” indicates such an error plotted in the case that the capacitancevalue of the externally-connectable capacitor Ci is equal to 0.07 μF.Also, a dotted line “c” indicates such an error plotted in the case thatthe capacitance value of the externally-connectable capacitor Ci isequal to 0.05 μF.

[0011] It should be noted that the relationship of FIG. 21 is obtainedunder the below-mentioned condition: That is, a range of an operationvoltage is selected to be zero V to +0.5 V and −0.5 V; sampling time ofan analog input voltage is selected to be 3.2 μS; an equivalentcapacitance value (Ca+Cs+Cd) defined from an analog input terminal AINup to a sample-and-hold circuit is selected to be approximately 50 pF;and further, both an equivalent impedance of an analog switch SWc of amultiplexer and another equivalent impedance of a sampling switch SWsown such a characteristic represented in FIG. 22. In FIG. 22, a curveindicated as a symbol “WORST” denotes an impedance of such an elementhaving the largest fluctuation. Also, a curve indicated as a symbol“TYP” denotes an impedance of a typical element.

[0012] The following fact can be seen from FIG. 21. That is, the largerthe capacitance values of the externally-connectable capacitors Ci1 toCin of the analog input terminals AIN1 to AINn become, the smaller theerror is decreased. However, when the capacitance values of theexternally-connectable capacitors Ci1 to Cin are increased in anautomobile control system, the following fact could be revealed. Thatis, there is a certain possibility that precision of the A/D convertingoperation is lowered. Under such a circumstance, the Inventors of thepresent invention has considered this reason of lowering of the A/Dconverting precision. As a result, the following fact can be revealed:

[0013] In such a system which is operated under the worst environmentwhere external noise is increased similar to a control system of anautomobile, since an S/N (signal-to-noise) ratio of a sensor functioningas an analog signal source is improved, a series resistor (resistancevalues of several tens to several hundreds KΩ) may be arranged betweenan analog input terminal of an A/D converter and a signal of the sensorso as to constitute a filter circuit. Alternatively, such a sensorcircuit having several tens to several hundreds KΩ as an overallimpedance of an analog signal source may be employed. As a consequence,the Inventors of the present invention could predict that a responsecharacteristic of an input voltage in this control system may cause theabove-explained reason, and firstly, have considered the responsecharacteristic of this control system with respect to the input voltage.Concretely speaking, while the Inventors of the present inventionconsidered that a voltage VA(i) which is entered into an analog inputterminal AIN(i) is changed in an exponential manner as represented inFIG. 23, such a formula capable of expressing a temporal change of thisinputted analog voltage was obtained. This formula is given as follows:

VA(i)=Vin(T−1)+ΔV(1−e(−t/(Ci·Rin)))   (2)

[0014] In this formula (2), symbol “Vin(T−1)” shows a voltage appearedbefore the analog input voltage VA(i) is changed.

[0015] Next, under such a condition that the error Ω(LSB) of the analoginput voltage Vin is equal to 0.1, the reference voltage Vref of the A/Dconverting circuit is equal to 5.0 V, and also the capacitance value ofthe externally-connectable capacitor Ci is equal to 0.1 μF, such a delaytime was calculated while the internal impedance value Rin of the signalsource was changed and also the magnitude of the changed voltage “ΔV”was changed. This delay time is defined by that the input voltage Vin isreached to 0.1(LSB) lower than a final voltage, namely is reached up to0.5 mV. The calculation result of this delay time is graphicallyrepresented in FIG. 24. As apparent from FIG. 24, the following fact canbe revealed. That is, the larger the internal impedance Rin of thesignal source is increased and also the higher the changed voltage “ΔV”becomes, the longer the delay time is increased.

[0016] Also, as apparent from the above-explained formula(1), since achange in the input voltages Vin will depend upon such a time constant(CR) defined between the internal impedance Rin and theexternally-connected capacitor Ci of the analog input terminal, thelarger the capacitance value of this externally-connectable capacitor Ciis increased, the longer the delay time is prolonged (similar to theinternal impedance Rin). For example, in FIG. 24, a broken line “C”which represents a delay time-to-changed voltage characteristic in thecase that the internal impedance Rin is equal to 5 kΩ may become equalto another broken line “B” which indicates a delay time-to-changedvoltage characteristic in such a case that the internal impedance Rin isequal to 10 kΩ when the capacitance value of the externally-connectablecapacitor Ci becomes 0.2 μF. This first-mentioned broken line “C” maybecome equal to a further broken line “A” which represents a delaytime-to-changed voltage characteristic in the case that the internalimpedance Rin is equal to 20 kΩ when the capacitance value of theexternally-connectable capacitor Ci becomes 0.4 μF.

[0017] On the other hand, even in a control system used under such anenvironment that a large number of external noise is present (similar inan automobile control system), it is practically difficult that internalimpedances of all of sensors functioning as an analog signal source arereduced. In other words, in such an automobile control system, such asensor whose internal impedance is high must be necessarily employed.Also, because of a distance relationship between a setting position of asensor and a setting position of an A/D converter, a series resistorfunctioning as an external-noise-problem solution is provided in frontof an analog input terminal, so that an impedance of a signal source isincreased. However, as explained above, if the capacitance value of theexternally-connectable capacitor Ci is increased, then delay time isprolonged. As a result, this capacitance value of theexternally-connectable capacitor Ci cannot be excessively increased. Asa consequence, in such a control system, the capacitance value of theexternally-connectable capacitor Ci must be made small in the case thata sampling period should be prolonged so as to secure A/D convertingprecision, or conversely in such a case that the sampling period isshortened so as to secure a favorable response characteristic to someextent. Accordingly, there is such a problem that the A/D convertingprecision would be sacrificed.

[0018] An object of the present invention is therefore to provide bothan analog switch and an analog multiplexer, by which an electron chargewhich has been stored in a stray capacitance before a switch is switcheddoes not give an adverse influence to a level of a subsequent analoginput voltage which is entered thereinto after the switch is switched.

[0019] Another object of the present invention is to provide such an A/Dconverting circuit capable of A/D-converting an analog input voltage inhigh precision.

[0020] A further object of the present invention is to provide such anA/D converting circuit capable of performing a high-precision A/Dconverting operation, while a high response speed thereof can berealized.

[0021] A still further object of the present invention is to providesuch an A/D converting circuit capable of performing a high-precisionA/D converting operation without lowering an S/N ratio thereof.

[0022] Typical inventive ideas selected from inventive ideas disclosedin the present invention will be briefly described as follows:

[0023] An analog switch circuit is constituted by a transfer gate madeup of insulating gate type transistors, and a voltage follower connectedin parallel to the transistors, and the analog switch circuit isarranged in such a manner that after the voltage follower is broughtinto an active state, the transistors are conducted.

[0024] In accordance with the above-explained means, it is possible toavoid such a condition that an adverse influence caused by a potentialappeared on the output side is transferred via the analog switch broughtinto the ON state to the output side. In other words, electric chargeswhich have been stored in a stray capacitance provided on the outputside thereof before a switch is switched do not give an adverseinfluence to a level of such an analog input voltage which issubsequently entered after the switch has been switched.

[0025] Also, preferably, the above-described transfer gate is made as aCMOS transfer gate constituted by both a p-channel MOSFET and ann-channel MOSFET, which are connected in parallel to each other betweenan analog input terminal and an analog output terminal. Since the CMOStransfer gate is employed, not only a signal having a high potential,but also a signal having a low potential can be transferred withoutlowering a signal level.

[0026] Furthermore, the above-described insulating gate type transistoris preferably constituted by two transistors, the channels of which areconnected in a series connection mode. Also, these two transistors areformed on different well regions which are formed on a semiconductorsubstrate. Since the analog switch circuit is constituted by the twotransistors connected in the series connection mode, it is possible toavoid such an operation that a change in analog input signals istransferred to the output terminal side via a stray capacitance producedbetween sources and drains of these transistors. Also, since the twotransistors connected in the series connection mode are formed on thedifferent well regions, the following operation can be prevented. Thatis, a current is captured from the output terminal side by minoritycarriers which are produced in well regions in such a case that anegative voltage is applied to the input terminal, so that the potentialis changed, which can be avoided by this analog switch circuit.

[0027] A multiplexer circuit, according to the present invention, issuch that a plurality of analog switch circuits are constituted by aplurality of transfer gates each made up of an insulating gate typetransistor, and voltage followers connected in parallel to channels ofthese transistors; and one terminal of these analog switch circuits areconnected to a plurality of analog input terminals, whereas otherterminals of these analog switch circuits are connected to a commonoutput terminal. When any one of these analog switch circuits isselectively turned ON, the voltage follower of this relevant switchcircuit is first brought into an active state, and thereafter, theinsulating type transistor is set to the conductive state.

[0028] In accordance with the above-described means, it is possible toavoid such a phenomenon that an adverse influence caused by a potentialappeared on the output side via the analog switch which is brought intothe ON state is transferred to the side of the input terminal. Also,electric charges which have been stored in a stray capacitance providedon the output side thereof before the multiplexer circuit is switched donot give an adverse influence to a level of such an analog input voltagewhich is subsequently entered after the multiplexer circuit has beenswitched.

[0029] Also in this multiplexer circuit, the above-explained transfergate may be preferably made as a CMOS transfer gate constituted by botha p-channel MOSFET and an n-channel MOSFET, which are connected inparallel to each other between the analog input terminal and the analogoutput terminal. Furthermore, the above-described insulating gate typetransistor is preferably constituted by two transistors, the channels ofwhich are connected in a series connection mode. Also, these twotransistors are formed on different well regions which are formed on asemiconductor substrate.

[0030] Also, an A/D converting circuit, according to the presentinvention, is featured by employing a sample-and-hold circuit which isconstituted by an analog switch circuit and a sample-and-hold circuitmade up of a sampling capacitor. This analog switch circuit is arrangedby an insulating gate type transistor, and a voltage follower which isconnected in parallel to a channel of this insulating gate typetransistor. The above-explained sampling capacitor is connected to anoutput terminal of the voltage follower circuit.

[0031] In accordance with the above-described means, it is possible toavoid such a phenomenon that an adverse influence caused by a potentialappeared on the output side via the analog switch circuit which isbrought into the ON state is transferred to the side of the inputterminal. Also, the analog input signal can be sampled in highprecision. As a result, the high-precision A/D converting operation canbe carried out. Further, even when a capacitance value of anexternally-connectable capacitor is decreased which is connected to theanalog input terminal, the higher A/D converting precision can beachieved. Therefore, even when such an analog signal source having ahigh internal impedance is employed, namely even when such an analogsignal source capable of withstanding noise aspects is used, propagationdelay time of an input signal can be shortened. As a consequence, insuch a system in which a plurality of analog signals are A/D-convertedby a single A/D converting circuit in the time divisional manner, whileusing the multiplexer, the sampling period can be shortened withoutsacrificing the A/D conversion precision, and also, the A/D convertingoperation with the better response characteristic can be carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above-described objects and other objects, and novel featuresof the present invention will become apparent from descriptions of thepresent specification and the accompanying drawings, in which:

[0033]FIG. 1 is a circuit diagram for representing an analog switchcircuit according to a first embodiment of the present invention;

[0034]FIG. 2 is a timing chart for representing timing of a controlsignal of the analog switch circuit according to the first embodiment;

[0035]FIG. 3 is a timing chart for showing another example of a controlsignal of the analog switch circuit according to the first embodiment;

[0036]FIG. 4 is a circuit diagram for showing an analog switch circuitaccording to a second embodiment of the present invention;

[0037]FIG. 5 is a circuit diagram for indicating an analog switchcircuit according to a third embodiment of the present invention;

[0038]FIG. 6 is a circuit diagram for indicating an analog switchcircuit according to a fourth embodiment of the present invention;

[0039]FIG. 7 is a circuit diagram for representing a concrete circuitexample of a voltage follower amplifier;

[0040]FIGS. 8A and 8B are a sectional view for showing an example ofsectional structures of n-channel MOSFETs Qn1 and Qn2 connected in aseries connection mode, which constitute a CMOS transfer gate TMG of ananalog switch circuit and a sectional view of a typical m-channel MOSFETfor comparison, respectively;

[0041]FIG. 9 is a circuit diagram for indicating a sample-and-holdcircuit for an analog voltage, according to a first embodiment of thepresent invention;

[0042]FIG. 10 is a circuit diagram for indicating a sample-and-holdcircuit for an analog voltage, according to a second embodiment of thepresent invention;

[0043]FIG. 11 is a circuit diagram for indicating a sample-and-holdcircuit for an analog voltage, according to a third embodiment of thepresent invention;

[0044]FIG. 12 is a circuit diagram for indicating an analog multiplexercircuit according to an embodiment of the present invention;

[0045]FIG. 13 is a timing chart for showing timing of a control signalof the analog multiplexer circuit according to this embodiment;

[0046]FIG. 14 is a circuit for representing an analog multiplexercircuit according to another embodiment of the present invention;

[0047]FIG. 15 is a circuit diagram for representing an A/D convertingcircuit according to an embodiment of the present invention;

[0048]FIG. 16 is a timing chart for representing an example of thecontrol timing of the A/D converting circuit according to the presentinvention;

[0049]FIG. 17 is a timing chart for representing another example of thecontrol timing of the A/D converting circuit according to the presentinvention;

[0050]FIG. 18 is a graphic representation for representing arelationship between an internal impedance of an analog signal sourceemployed in an A/D converting circuit, and an error δ(LSB) of an analoginput voltage sampled by a sample-and-hold circuit, to which the presentinvention is applied;

[0051]FIG. 19 is a block diagram for showing a structural example of asingle-chip microcomputer functioning as one example of a suitablesystem LSI, while using the A/D converting circuit according to thepresent invention;

[0052]FIG. 20 is a block diagram for indicating a schematic arrangementof a system which A/D-converts a plurality of analog signals in a timedivisional manner;

[0053]FIG. 21 is a graphic representation for showing the relationshipbetween the internal impedance Rin of the analog signal source and theerror δ(LSB) of the analog input voltage to be sampled in theconventional A/D converting circuit operated in the time divisionalsystem;

[0054]FIG. 22 is a graphic representation for indicating therelationship among the impedance of the MOSFET which constitutes themultiplexer circuit, the impedance of the MOSFET which constitutes thesample-and-hole circuit, and the analog input voltage Vin;

[0055]FIG. 23 is a diagram for indicating the condition of the voltagechanges at the terminal into which the analog signal is inputted whenthe voltage change of “ΔV” is produced in the analog signal source; and

[0056]FIG. 24 is a graphic representation for representing therelationship between the magnitude of the voltage change “ΔV” in theanalog signal source, and the propagation delay time of the terminal towhich the analog signal is inputted when this voltage change occurs,while the internal impedance of the analog signal source is used as theparameter.

DESCRIPTION OF THE EMBODIMENTS

[0057] Referring now to drawings, embodiments of the present inventionwill be described in detail.

[0058]FIG. 1 is an analog switch circuit according to a first embodimentof the present invention. In FIG. 1, symbol “AIN” shows an analog inputterminal; symbol “AOUT” denotes an analog output terminal; symbol “TMG”represents a transfer gate connected between the above-described analoginput terminal AIN and the analog output terminal AOUT; symbol “VFA”indicates a voltage follower amplifier which is connected in parallel tothe above-described CMOS transfer gate TMG between the analog inputterminal AIN and the analog output terminal AOUT; and symbols “INV1” and“INV2” denote inverters for inverting a control signal CS1. The transfergate TMG is constituted by such a CMOS transfer gate in which ap-channel MOSFET Qp1 is connected in parallel to an n-channel MOSFETQn1.

[0059] Then, both the p-channel MOSFET Qp1 and the n-channel MOSFET Qn1,which constitute the above-explained CMOS transfer gate TMG, aresimultaneously brought into either ON states or OFF states by supplyingone pair of control signals “CS1” and “/CS1” to gate terminals of thesep-channel/n-channel MOSFETs Qp1/Qn1. This control signal CS1 owns aphase opposite to that of the remaining control signal /CS1. The voltagefollower amplifier VFA is constituted by such a differential amplifyingcircuit, the output terminal of which is connected to the invertinginput terminal thereof. In response to a control signal “CS0”, thisvoltage follower amplifier VFA may be controlled under either an activestate or a non-active state (inactive state). Under this active state,the voltage follower amplifier VFA is operated in such a manner that avoltage Vin having the same level as a level of an input voltage may beoutputted to the output terminal AOUT.

[0060] The analog switch circuit according to this embodiment iscontrolled in response to the control signals CS1 and CS0, which aresupplied thereof at such timing as shown in FIG. 2. Concretely speaking,first of all, the voltage follower amplifier VFA is brought into theactive state at timing “t1” in response to the control signal CS0, andthen, the output voltage of this voltage follower amplifier VFA ischanged into the same level as that of the input voltage Vin.Subsequently, in response to the control signal CS1, both the p-channelMOSFET Qp1 and the n-channel MOSFET Qn1, which constitute the CMOStransfer gate TMG, are simultaneously brought into ON states at timing“t2”, so that this CMOS transfer gate TMG transfers the analog inputvoltage Vin entered into the analog input terminal AIN to the analogoutput terminal AOUT.

[0061] In the conventional analog switch circuit having no such avoltage follower amplifier VFA, when the potential difference betweenthe analog input voltage Vin and the voltage which has been charged intothe stray capacitance existed on the side of the output terminal thereofis high, the level of the voltage transferred by the analog switch isadversely influenced before both the p-channel MOSFET Qp1 and then-channel MOSFET Qn1, which constitute the CMOS transfer gate TMG, areturned ON. To the contrary, in the analog switch circuit according tothis embodiment, the voltage follower amplifier VFA is firstly broughtinto the active state, and then, the output voltage of this voltagefollower amplifier VFA is changed toward the same level as that of theinput voltage. As a result, both the p-channel MOSFET Qp1 and then-channel MOSFET Qn1, which constitute the CMOS transfer gate TMG, areturned ON.

[0062] As a result, just before the CMOS transfer gate TMG is turned ON,the potential difference between the analog input voltage Vin and thecharged voltage of the stray capacitance existed on the side of theoutput terminal may become very small. Even when the CMOS transfer gateTMG is turned ON under this condition, substantially no adverseinfluence is given to the level of the voltage which is transferred bythe switch. Also, after the CMOS transfer gate TMG has been turned ON,since the voltage follower amplifier VFA is brought into the non-activestate, even when the voltage follower amplifier VFA owns an offset dueto fluctuations of structural elements, the correct input voltage levelcan be transferred to the output terminal by the CMOS transfer gate TMG.

[0063] It should be noted that in the timing chart shown in FIG. 2, aportion of the high-level period of the control signal CS0 is overlappedwith a portion of the high-level period of the control signal CS1.Alternatively, as represented in FIG. 3(B) and FIG. 3(C), the CMOStransfer gate TMG may be controlled in response to such signals, thehigh-level periods of which are not overlapped with each other. Also,the control signal CS0 may be made as a control signal having suchtiming that this control signal falls to a low level before the voltagefollower amplifier VFA sets a level of an output voltage to such a levelwhich is completely equal to a level of an input voltage, by which thevoltage follower amplifier VFA is brought into the non-active state.

[0064] Furthermore, as indicated by a broken line, a timing generatingcircuit TG may be provided in the vicinity of the analog switch circuit.This timing generating circuit TG generates both such a control signalCS0 as indicated in FIG. 3(B), and also another control signal CS1 asindicated in FIG. 3(C) based upon such a control signal as indicated inFIG. 3(A) which is applied to the conventional analog switch. Thiscontrol signal CS0 controls the voltage follower amplifier VFA of thefirst embodiment of FIG. 1, whereas the control signal CS1 controls theCMOS transfer gate TMG thereof. In this alternative case, a user mayperform the completely same use method as that of the conventionalanalog switch circuit, so that the design work load given to the usermay be reduced.

[0065]FIG. 4 shows an analog switch circuit according to a secondembodiment of the present invention. This analog switch circuit of thesecond embodiment is arranged as follows: That is, a p-channel MOSFETQp2 and an n-channel MOSFET Qn2 are newly employed which are connectedin series to the p-channel MOSFET Qp1 and the n-channel MOSFET Qn1,which constitute the CMOS transfer gate TMG employed in the analogswitch circuit of the first embodiment of FIG. 1. Thesep-channel/n-channel MOSFETs Qp2/Qn2 are simultaneously turned ON/OFF inresponse to the same control signals CS1 and /CS1 as those of therespective MOSFETs Qp1 and Qn1, respectively.

[0066] This analog switch circuit of this second embodiment iscompletely identical to the above-mentioned analog switch circuit of thefirst embodiment shown in FIG. 1 in terms of operation. In the analogswitch circuit of the first embodiment shown in FIG. 1, when the inputvoltage Vin is rapidly changed, there is such a risk that this change inthe input voltages is transferred to the output terminal via the straycapacitances produced between the sources and the drains of thep-channel MOSFET Qp1 and the n-channel MOSFET Qn1 respectively, whichconstitute the CMOS transfer gate TMG. To the contrary, in the analogswitch circuit of the second embodiment shown in FIG. 4, the p-channelMOSFET Qp2 is series-connected to the p-channel MOSFET Qp1, and also,the n-channel MOSFET Qn2 is series-connected to the n-channel MOSFETQn1, which constitute the CMOS transfer gate TMG. As a result, thiscircuit may be equivalent to such a circuit that two stray capacitancesproduced between the sources and the drains of these p-channel/n-channelMOSFETs are series-connected between the input terminal AIN and theoutput terminal AOUT. Therefore, there is such a merit that even whenthe input voltage Vin is rapidly changed, this voltage change can behardly transferred to the output terminal side.

[0067] As explained in the second embodiment of FIG. 4, in the case ofsuch an analog switch circuit in which MOSFETs are series-connected toeach other, an input resistance of this analog switch circuit may becomehigher than an input resistance of another analog switch constituted bya single MOSFET. However, since the input resistance may be reduced byconducting such a measure that gate widths of these MOSFETs are madewider than a gate width of such a single MOSFET, there is no problem.

[0068]FIG. 5 shows an analog switch circuit according to a thirdembodiment of the present invention. This analog switch circuit of thethird embodiment is arranged as follows: That is, an n-channel MOSFETQna1 is connected between the ground point and a coupling node Ni of theseries-connected p-channel MOSFETs Qp1 and Qp2, and also, a p-channelMOSFET Qpa1 is connected between the power supply voltage Vcc andanother coupling node N2 of the series-connected n-channel MOSFETs Qn1and Qn2, which constitute the CMOS transfer gate TMG employed in theanalog switch circuit of the second embodiment shown in FIG. 4.

[0069] Then, the p-channel MOSFET Qpa1 connected to the above-describednode N1 is turned ON/OFF in response to the control signal CS1 used forthe n-channel MOSFETs Qn1 and Qn2. Also, n-channel the MOSFET Qna1connected to the node N2 is turned ON/OFF in response to the controlsignal /CS1 used for the p-channel MOSFETs Qp1 and Qp2 in acomplementary manner with respect to the p-channel MOSFETs Qp1 and Qn1.In other words, when the MOSFETs Qp1, Qp2, Qn1, and Qn2 are turned ON,these MOSFETs Qpa1 and Qna1 are turned OFF, whereas when the MOSFETsQp1, Qp2, Qn1, and Qn2 are turned OFF, these MOSFETs Qp1 and Qna1 areturned ON. As a result, when the analog switch circuit is set under OFFstate, it is possible to avoid that both the coupling node N1 of theseries-connected p-channel MOSFETs Qp1 and Qp2, and the coupling node N2of the series-connected n-channel MOSFETs Qn1 and Qn2 are brought intofloating states in view of the potential aspect. Since these nodes N1and N2 are fixed to the ground potential and the power supply voltageVcc, respectively, such an operation can be prevented. That is, whilethe analog switch circuit is under OFF state, the change in the inputvoltages is transferred to the output terminal via the straycapacitances between the sources and the drains, and also via theparastic bipolar transistor structural portion.

[0070] It should be noted that in the third embodiment of FIG. 5, thenode N1 is fixed to the ground potential, and the node N2 is fixed tothe power supply voltage Vcc. Alternatively, the node N1 may be fixed tothe power supply voltage Vcc, and the node N2 is fixed to the groundpotential. Also, both the node N1 and the node N2 may be fixed to thesame potential. Furthermore, as explained later, since device structuralarrangements are applied to the analog switch circuit, a change in inputvoltages may be hardly transferred to the output terminal.

[0071]FIG. 6 shows an analog switch circuit according to a fourthembodiment of the present invention. This analog switch circuit of thefourth embodiment is arranged as follows: Instead of controlling theMOSFETs Qpa1 and Qna1 by the control signals CS1 and /CS1 used for theMOSFETs Qp1, Qp2, Qn1, and Qn2, which are provided in order to fix thepotentials at the node N1 and the node N2 employed in the thirdembodiment of FIG. 5, these MOSFETs Qpa1 and Qna1 are turned ON by usingother control signals CS1 and /CS2 when the MOSFETs Qp1, Qp2, Qn1, andQn2 are turned OFF, whereas these MOSFETs Qpa1 and Qna1 are turned OFFwhen the MOSFETs Qp1, Qp2, Qn1, and Qn2 are turned ON.

[0072]FIG. 7 represents a concrete circuit example of theabove-described voltage follower amplifier VFA.

[0073] As indicated in FIG. 7, the voltage follower amplifier VFA ofthis embodiment is constituted by a differential input stage 11, anoutput stage 12, and a bias stage 13. Among these circuit stages, thedifferential input stage 11 is constructed of a general-purposedifferential amplifying circuit. This differential amplifying circuit isconstituted by differential MOSFETs Q1 and Q2, the sources of which arecommonly connected to each other; active load MOSFETs Q3 and Q4 whichare connected to the drain sides of the differential MOSFETs Q1 and Q2;and an MOSFET Q5 used for a current source. This MOSFET Q5 is connectedbetween the ground point and the commonly-connected sources of thedifferential MOSFETs Q1 and Q2. Then, the input voltage Vin is appliedto a gate terminal of the differential MOSFET Q2.

[0074] The output stage 12 is arranged by an output MOSFET Q6 in which apotential of an inverting-sided output node of the differential inputstage 11 is received by the gate thereof; an MOSFET Q7 used for acurrent source, which is connected in series to this output MOSFET Q6;and a switch MOSFET Q8. This switch MOSFET Q8 is connected between thegate of the output MOSFET Q6 and the power supply voltage Vcc, and isturned ON/OFF in response to the control signal CS0. Then, a voltage ofthe drain side of the output MOSFET Q6 is fed back to the gate of thedifferential MOSFET Q1 of the differential input stage 11. As a result,the differential input stage 11 is operated in such a manner that theinput voltage Vin of the MOSFET Q2 is made coincident with the gatevoltage of the MOSFET Q1, and also, such a voltage Vout having the samelevel as that of the input voltage Vin is outputted from the outputstage 12.

[0075] The bias stage 13 is arranged by a p-channel MOSFET Q9series-connected between the power supply voltage Vcc and the groundpoint; a resistor R1; an n-channel MOSFET Q10 which is connected in adiode manner; and also, an n-channel MOSFET Q11. This n-channel MOSFETQ11 is connected between a drain of the n-channel MOSFET Q10 and theground point, and is turned ON/OFF in response to the control signal/CS0. While the control signal /CS0 is also applied to a gate of theabove-explained MOSFET Q9, both the p-channel MOSFET Q9 and then-channel MOSFET Q11 are ON/OFF-controlled in the complementary manner.When the level of the control signal /CS0 becomes a low level, theMOSFET Q9 is turned ON, so that a voltage Vb is produced, and thisvoltage Vb is defined by dividing the power supply voltage Vcc by such aratio among an ON-resistance of the MOSFET Q9, a resistance value of theresistor R1, and a conductance of the MOSFET Q10. This voltage Vb isapplied as a bias voltage to both the gate of the MOSFET Q5 for thecurrent source employed in the differential input stage 11, and the gateof the MOSFET Q7 for the current source employed in the output stage 12.When the bias voltage Vb is applied to these gates, currents will flowthrough these MOSFETs Q5/Q7, so that the circuit is brought into theactive state.

[0076] Also, when the level of the above-explained control signal /CS0is set to a high level, the MOSFET Q9 is turned OFF and the MOSFET Q11is turned OFF, so that the bias stage 13 stops the generation of thebias voltage Vb. As a result, both the current flowing through theMOSFET Q5 for the current source employed in the differential inputstage 11, and the current flowing through the MOSFET Q7 for the currentsource employed in the output stage 12 are interrupted, and the voltagefollower amplifier VFA stops the operation thereof.

[0077]FIG. 8A indicates an embodiment of a structural diagram of theCMOS transfer gate TMG in a device level, employed in the analog switchcircuit of FIG. 4. It should be understood that the device design ofthis embodiment may be applied not only to the device constructions ofthe CMOS transfer gates TMG employed in the analog switch circuitsaccording to the third embodiment of FIG. 5 and the fourth embodiment ofFIG. 6.

[0078]FIG. 8A illustratively shows a sectional structure of then-channel MOSFETs Qn1 and Qn2 among the p-channel MOSFETs Qp1/Qp2 andthe n-channel MOSFETs Qn1/Qn2, which constitute the CMOS transfer gateTMG.

[0079] As indicated in FIG. 8A, although no specific limitation is madein this embodiment mode, a relatively deep n-type well region 110 isformed on the side of a major surface of a p-type semiconductorsubstrate 100, and furthermore, two p-type well regions 121 and 122 areformed on the side of a surface of this n-type well region 110. Then,diffusion layers 211 and 212, and a gate electrode 213 are formed in thesurface of one p-type well region 121. The diffusion layers 211 and 212function as the source region and the drain region of the n-channelMOSFET Qn1 which constitutes the above-explained CMOS transfer gate TMG.Also, diffusion layers 221 and 222, and a gate electrode 223 are formedin the surface of the other p-type well region 122. The diffusion layers221 and 222 function as the source region and the drain region of then-channel MOSFET Qn2 which constitutes the above-explained CMOS transfergate TMG.

[0080] The diffusion layer 211 of the n-channel MOSFET Qn1 is connectedto the analog input terminal AIN, and also the diffusion layer 212thereof is connected via a wiring line 231 made of aluminum and the liketo the diffusion layer 221. Also, the diffusion layer 222 of then-channel MOSFET Qn2 is connected via another wiring line 232 to theoutput terminal. Furthermore, while the ground potential is applied tothe above-described p-channel semiconductor substrate 100, the powersupply voltage Vcc is applied to the n-type well region 110, and alsothe ground potential is applied to both the p-type well regions 121 and122, so that the PN junction is biased under reverse biasing condition.

[0081]FIG. 8B illustratively shows a sectional structure of a typicaln-channel MOSFETs connected in a series connection mode, for the sake ofa comparison. In this drawing, reference numeral 120 shows a p-type wellregion; reference numerals 211 and 212 indicate diffusion layersfunctioning as the source region and the drain region of the n-channelMOSFET Qn1; and also reference numerals 221 and 222 represent diffusionsfunctioning as the source region and the drain region of the n-channelMOSFET Qn2. As indicated in this drawing, in the typical n-channelMOSFETs connected in the serial connection mode, two MOSFETs are oftenformed on a single p-type well region 120.

[0082] The Inventors of the present invention could find out thebelow-mentioned problem: That is, in such a case that such n-channelMOSFETs connected in the series connection mode, which has been formedon the single p-type well region 120, are used as the n-channel MOSFETsQn1 and Qn2 which constitute the CMOS transfer gate TMG of FIG. 4, if anegative potential is applied to the analog input terminal AIN due tosome reason, then the output voltage is changed.

[0083] As a result of their investigations as to this reason, theInventors of the present invention could come to such a conclusion thatthis reason could be explained based upon the below-mention assumption.In other words, as to such an analog switch which employs the MOSFETshaving such a structure as shown in FIG. 8B, in the case that thenegative potential is entered into the analog input terminal AIN whichis connected to the diffusion layer 211 of the n-channel MOSFET Qn1 dueto some reason, the PN junction between the p-type well region 120 andthe n-type diffusion layer 211 is biased in the forward bias mode, sothat a current will flow toward the analog input terminal AIN. As aresult, minority carriers (electrons) are produced in the vicinity ofthe diffusion layer 211 of the p-type well region 120, and then, theseminority carriers are diffused to an entire region of the p-type wellregion 120. As a result, electron charges will flow from the diffusionlayer 222 which is connected to the side of the output terminal intothis p-type well region 120 in order to neutralize these minoritycarriers, so that the output voltage is changed.

[0084] On the other hand, as to such an analog switch which employs theMOSFETs having the structure as shown in FIG. 8A, since the n-channelMOSFETs Qn1 and Qn2 are formed on the separated p-type well regions 121and 122 respectively, even when the following phenomenon occurs, theseminority carriers are not diffused into the p-type well region 122. Thatis to say, even when the negative potential is entered into the analoginput terminal AIN which is connected to the diffusion layer 211 of then-channel MOSFET Qn1, so that a current will flow toward the analoginput terminal AIN, and thus, minority carriers (electrons) are producedin the vicinity of the diffusion layer 211 of the p-type well region121, these minority carriers are not diffused to the p-type well region122 of the MOSFET Qn2. Accordingly, it is possible to avoid such acondition that electronic charges will flow from the diffusion layer 222which is connected to the side of the output terminal into this p-typewell region 122 in order to neutralize these minority carriers, so thatthe output voltage is changed.

[0085] On the other hand, considering such a case that a relatively deeppotential is entered into the analog input terminal, even when theMOSFETs Qn1 and Qn2 are separately formed on the p-type well regions 121and 122 as represented in FIG. 8A, the following operation may beconceived. That is, a current may flow to the analog input terminal AIN,so that a potential appeared in the vicinity of the diffusion layer 211of the p-type well region 120 is lowered, a bipolar transistor which isparasitically formed in the diffusion layers 211 and 212 of then-channel MOSFET Qn1 becomes conduct. Then, this lowered potential istransferred to the diffusion layer 221 via a wiring line which is usedto connect between the n-channel MOSFETs Qn1 and Qn2, and thus, abipolar transistor which is parasitically formed in the diffusion layers221 and 222 of the n-channel MOSFET Qn2 becomes conduct, so that acurrent may flow and therefore, an output voltage may be changed.

[0086] However, as explained in the embodiments of FIG. 5 and FIG. 6,the potential at the diffusion layer 221 of the MOSFET Qn2 is fixed insuch a case that while the MOSFET Qp2 is connected between the powersupply voltage Vcc and the connection node N2 (namely, wiring line 140of FIG. 8B) between the n-channel MOSFETs Qn1 and Qn2, when then-channel MOSFETs Qn1 and Qn2 are turned OFF, the p-channel MOSFET Qpa1is turned ON so as to fix the potential at the connection node N2 to thepower supply voltage Vcc. As a consequence, the following phenomenon canbe more effectively prevented. That is, since the negative potential isentered into the analog input terminal AIN, the minority carriers(electrons) are produced in the vicinity of the diffusion layer 211 ofthe p-type well region 120, and then, these minority carriers arediffused into the p-type well region 120. Even when the potential at thediffusion layer 212 of the n-channel MOSFET Qn1 is lowered, this loweredpotential is transferred to the diffusion layer 221 of the n-channeltransistor Qn2, so that the bipolar transistor which is parasiticallyproduced in the diffusion layers 221 and 222 of the n-channel MOSFET Qn2is turned ON, and therefore, the potential appeared on the output sideis changed, which can be prevented by the above-described embodiments.

[0087]FIG. 8A represents such sectional structures of the n-channelMOSFETs Qn1 and Qn2 among the p-channel MOSFETs Qp1/Qp2 and then-channel MOSFETs Qn1/Qn2, which constitute the CMOS transfer gate TMG.Alternatively, even when the p-channel MOSFETs Qp1 and Qp2 similarlyemploy such a structure that the well regions are separated, a similareffect may be achieved.

[0088]FIG. 9 represents a sample-and-hold circuit for an analog voltage,according to a first embodiment of the present invention. Thesample-and-hold circuit of this first embodiment owns such aconstruction that while the analog switch circuit of the firstembodiment shown in FIG. 1 is used, a sampling capacitor Cs is connectedbetween the output terminal of this analog switch circuit and the groundpoint.

[0089] Similarly to the analog switch circuit of the first embodimentshown in FIG. 1, also, in the sample-and-hold circuit of this firstembodiment, the voltage follower amplifier VFA is firstly brought intothe active state, and the output voltage thereof is changed into thesame level as the level of the input voltage. Thereafter, both thep-channel MOSFET Qp1 and the n-channel MOSFET Qn1 are turned ON, whichconstitute the CMOS transfer gate TMG. Then, the analog input voltageVin is conducted into the sampling capacitor Cs, whereas when both thep-channel MOSFET Qp1 and the n-channel MOSFET Qn1 are turned OFF, thecharged voltage of the sampling capacitor Cs is held.

[0090] In this sample-and-hold circuit of the first embodiment, when theCMOS transfer gate TMG is turned ON due to the immediately-beforeoperation of the voltage follower amplifier VFA, a potential differencebecomes very small, and this potential difference is defined between theinput voltage Vin and the charged voltage of the stray capacitanceexisted on the side of the output terminal. As a consequence, even whenthe CMOS transfer gate TMG is turned ON under this condition, the levelof the input voltage which is transferred by the analog switch is notsubstantially and adversely influenced by the charged electrons of thestray capacitance. Further, after the CMOS transfer gate TMG has beenturned ON, or just before this CMOS transfer gate TMG is turned ON, thevoltage follower amplifier VFA is brought into the non-active state. Asa result, even when the voltage follower amplifier VFA owns the offsetdue to the fluctuation in the structural elements, the input voltage canbe correctly transferred by way of the CMOS transfer gate TMG.

[0091] It should be noted that similar to the embodiment shown in FIG.3, also in the embodiment indicated in FIG. 9, it may become effectiveto employ such a timing generating circuit. This timing generatingcircuit generates the control signals CS0 and CS1 used to control theMOSFETs Qp1 and Qn1 which constitute the analog switch circuit, and alsothe voltage follower amplifier VFA in response to the supplied controlsignal CS. As a consequence, it is possible to achieve such a similareffect that the provision of this timing generating circuit may reducethe designing work load of the user.

[0092]FIG. 10 represents a sample-and-hold circuit for an analogvoltage, according to a second embodiment of the present invention. Thesample-and-hold circuit of this second embodiment owns such aconstruction that while the analog switch circuit of the secondembodiment shown in FIG. 5 is used, a sampling capacitor Cs is connectedbetween the output terminal of this analog switch circuit and the groundpoint.

[0093] Similarly to the analog switch circuit of the second embodimentshown in FIG. 5, also, in the sample-and-hold circuit of this secondembodiment, the p-channel MOSFET Qp2 is connected in series to thep-channel MOSFET Qp1, and also the n-channel MOSFET Qn2 is connected inseries to the n-channel MOSFET Qn1, while both the p-channel/n-channelMOSFETs Qp1/Qn1 constitute the CMOS transfer gate TMG. As a result, thissample-and-hold circuit may be equivalent to such a circuit that twosets of stray capacitances produced between the sources and the drainsare series-connected between the input terminal AIN and the samplingcapacitance Cs. Therefore, there is such a merit that even when theinput voltage Vin is rapidly changed, this voltage change can be hardlytransferred to the sampling capacitor Cs.

[0094] Also, in this sample-and-hold circuit of the second embodiment,it is preferable to utilize such a circuit having the device structureas represented in FIG. 8 as the analog switch circuit. As a consequence,when the negative potential is inputted to the input terminal, theadverse influence caused by this negative potential can be hardlytransferred to the side of the sampling capacitance.

[0095]FIG. 11 represents a sample-and-hold circuit for an analogvoltage, according to a third embodiment of the present invention. Thesample-and-hold circuit of this third embodiment owns such aconstruction that while the analog switch circuit of the thirdembodiment shown in FIG. 6 is used, a sampling capacitor Cs is connectedbetween the output terminal of this analog switch circuit and the groundpoint.

[0096] Similarly to the analog switch circuit of the third embodimentshown in FIG. 6, also, in the sample-and-hold circuit of this thirdembodiment, it is possible to avoid such a condition that when theanalog switch circuit is brought into an OFF state, potential-fixingMOSFETs Qpa1 and Qna1 are turned ON, so that both the coupling node N1of the p-channel MOSFETs Qp1/Qp2 connected in the series connectionmode, and also, the coupling node N2 of the n-channel MOSFETs Qn1/Qn2are brought into the floating states in view of the potential aspect.Thus, the respective nodes N1 and N2 can be fixed to both the groundpotential and the power supply voltage Vcc, respectively. As a result,it is possible to suppress transferring of a change in input voltages tothe sampling capacitance Cs via both the stray capacitance producedbetween the source and the drain, and also the parasitic bipolartransistor structural element portion while the analog switch is turnedOFF.

[0097] Also, in this sample-and-hold circuit of the third embodiment, itis preferable to utilize such a circuit having the device structure asrepresented in FIG. 8 as the analog switch circuit. As a consequence,when the negative potential is inputted to the input terminal, theadverse influence caused by this negative potential can be hardlytransferred to the side of the sampling capacitance.

[0098]FIG. 12 represents an analog multiplexer circuit according to afirst embodiment of the present invention. This analog multiplexercircuit of the first embodiment owns such an arrangement that one end ofeach of the analog switch circuits ASW1, ASW2, - - - , ASWn of the firstembodiment shown in FIG. 1 are connected to each of the plural analoginput terminals AIN1, AIN2, - - - , AINn, and further, other ends ofthese analog switch circuits are connected to a common output terminalAout.

[0099] It should also be noted that elements to which symbols “TMG1” to“TMGn” are applied in FIG. 12 represent such CMOS transfer gates, eachof which is constituted by the p-channel MOSFET Qp1 and the n-channelMOSFET Qn1 in the first embodiment of FIG. 1. It should also beunderstood that these CMOS transfer gates TMG1 to TMGn are not limitedto the constructions shown in FIG. 1, but may be realized by employingsuch structures similar to those of the CMOS transfer gates TMG of theanalog switch circuits represented in FIG. 3, FIG. 5, and FIG. 6. As aresult, similar effects as those of the respective embodiments may beachieved.

[0100] The analog multiplexer circuit of this first embodiment isoperated as follows: That is, as indicated in a timing chart of FIG. 13,since a signal level of any one of control signals CS01, CS02, - - - ,CS0n and also control signals CS11, CS12, - - - , CS1n, which controlthe analog switch circuits ASW1, ASW2, - - - , ASWn is selectivelychanged into a high level, the corresponding analog switch circuitsASW1, ASW2, - - - , ASWn are sequentially brought into a conductingcondition, so that this analog multiplexer circuit transfers such asignal which is selected from signals supplied to the analog inputterminals AIN1, AIN2, - - - , AINn. Moreover, while a switch is broughtinto the conductive state, first of all, voltage follower amplifiers VFAmay become active in response to the control signals CS01, CS02, - - - ,CS0n, and then a potential appeared on the output side is approached toa level of an input potential. Thereafter, the CMOS transfer gates TMG1to TMGn are brought into conductive states in response to the controlsignals CS11, CS12, - - - , CS1n, so that this analog multiplexercircuit may transfer the correct input level.

[0101]FIG. 14 shows an analog multiplexer circuit according to a secondembodiment of the present invention. This analog multiplexer circuit ofthe second embodiment is manufactured as a semiconductor integratedcircuit (IC) on a single semiconductor chip in such a manner that adecoder circuit DEC is combined with such an analog multiplexer circuitMPX constructed as explained in the first embodiment of FIG. 12, andthese multiplexer/decoder circuits are formed on this singlesemiconductor chip. FIG. 14 represents an arrangement of a multiplexercircuit as one example, and this multiplexer circuit selects one of 8sorts of signals to output this selected signal.

[0102] In this drawing, symbols “S2”, “S1”, and “S0” indicate 3-bitselect signals for designating that any one of input signals isselected; symbol “CS0” represents a signal for applying such timing atwhich a voltage follower amplifier VFA is brought into an active state;and also symbol “CS1” shows a signal for applying such timing at which atransfer gate TMG made of MOSFETs is turned ON. In the case that such asystem is arranged with employment of an LSI (Large-Scale Integration)which contains either an A/D converting LSI or an A/D convertingcircuit, the multiplexer IC of this second embodiment is connected to afront stage of an analog input terminal of the A/D converting circuit,which may give a merit. Since this multiplexer IC is used, a user maymerely design such a circuit capable of generating only the 3-bit selectsignals S0 to S2, and further, one set of timing signals CS0 and CS1.However, this user no longer requires to design another circuit capableof producing the timing control signals for the respective channels. Asa result, there is such an advantage that the design work load given tothe user can be reduced.

[0103]FIG. 15 shows an A/D converting circuit of an embodiment by usingboth the analog multiplexer circuit and the sample-and-hold circuit, asexplained in the previous embodiment.

[0104] In FIG. 15, symbol “MPX” shows an analog multiplexer circuit forselecting one of plural analog input signals AIN0 to AINn; symbol “SH”represents a sample-and-hold circuit for sampling a selected analoginput signal; and also symbol “ADC” shows an A/D converting circuit forconverting a sampled analog signal into a digital signal correspondingthereto.

[0105] The A/D converting circuit ADC is constituted by a comparatorCMP, a converted-result register REG, a D/A converting circuit DAC, ashift register SFR, a control circuit CNT, and also a control registerCRG. The comparator CMP compares an input analog signal with a referencevoltage Vref. The converted-result register REG is, for example, a10-bit converted-result register for sequentially holding a convertedresult. The shift register SFR sequentially holds a 10-bit digitalsignal A/D-converted from each of the analog input signals. The controlcircuit CNT controls an entire circuit of this A/D converting circuitADC. The control register CRG is used to set such control information asa designation sequence and a conversion sequence with respect to acontrol mode of the control circuit CNT and an analog signal to beconverted.

[0106] In the sample-and-hold circuit SH employed in the A/D convertingcircuit ADC shown in FIG. 15, such a circuit shown in FIG. 9, FIG. 10,or FIG. 11 is used, whereas in the analog multiplexer circuit MPX, sucha circuit indicated in FIG. 12 is used. Then, further, in the analogswitch circuit employed in the circuit of FIG. 12, such a circuit shownin FIG. 1, FIG. 3, FIG. 5, or FIG. 6 is provided.

[0107] The control circuit CNT supplies the select signals S2, S1, S0for the analog input signal, and the control signal CS0, CS1 withrespect to the analog multiplex circuit MPX, and also supplies asampling timing signal “φs” with respect to the sample-and-hold circuitSH. In addition, this control circuit CNT controls the D/A convertingcircuit DAC based upon a comparison result obtained by the comparatorCMP, designates a bit used to hold a comparison result in theconverted-result register REG, controls the shift operation of the shiftregister SFR, and/or outputs a value stored in the shift register SFR toa bus BUS.

[0108] The D/A converting circuit DAC is constituted by a resistorladder RRD and a switch array SAL. This resistor ladder RRD divides areference voltage “Vref0” by using a plurality of resistors. The switcharray SAL selects any one of the subdivided reference voltages to applythe selected reference voltage to the comparator CMP.

[0109] The control circuit CNT first controls the comparator CMP tocompare such an analog signal which is selected and entered thereintowith a comparison voltage of Vref/2 so as to store this comparisonresult into the register REG, and switches a voltage into a voltage of 3Vref/4, which is applied from the D/A converting circuit DAC to thecomparator CMP when the selected analog signal is higher than thecomparison voltage of Vref/2 based upon the comparison result. On theother hand, when the selected analog signal is lower than the comparisonvoltage Vref/2, the control circuit CNT switches the voltage into avoltage of Vref/4, which is applied from the D/A converting circuit DACinto the comparator CMP.

[0110] While the control circuit CNT sequentially performs theabove-explained voltage comparison and the switching operation of thecomparison voltage and further holds the respective comparison resultsat the different bits of the register REG, the control circuit CNTgradually approximates such a comparison voltage to the analog inputsignal, and this comparison voltage is applied to the comparator CMP tothe analog input signal. When the final bit is set to the register REGby sequentially repeating the voltage comparing operation, for instance,a value of a 10 bit which is held in the register REG corresponds to adigital value obtained after the A/D converting operation.

[0111] In the A/D converting circuit of this embodiment, as representedin FIG. 16(A), the sampling process operation and the A/D convertingprocess operation are repeatedly carried out in an alternate manner.During a sampling time period, under such a condition that the analogswitch of the sample-and-hold circuit SH is turned ON in response tosuch a sampling signal “φs” shown in FIG. 16(B), any one of the voltagefollower amplifiers VFA employed in the analog multiplexer circuit MPXis brought into the active state in response to such select signals CS01to CS0n as indicated in FIG. 16(C), FIG. 16(E), FIG. 16(G) at a fronthalf sampling time period of each of the sampling time periods. Then, inresponse to such select signals CS1 to CS1n shown in FIG. 16(D), FIG.16(F), FIG. 16(H), the MOSFET is turned ON at a rear half time period ofeach of the sampling time periods, so that the analog input signal maybe transferred to the sample-and-hold circuit. Thereafter, both theselect signals CS01 to CS0n, CS11 to CS1n, and also the sampling signalSMP are brought into OFF states (low levels), so that while the sampledsignal is held by the sample-and-hold circuit SH, the A/D convertingcircuit is brought into the active state so as to execute the A/Dconverting process operation.

[0112] Furthermore, in the A/D converting circuit of this embodiment, asrepresented in FIG. 17, within such a time period “Tn” during which thisA/D converting circuit performs the A/D converting process operationwith respect to such an analog signal which is sampled in a certaincycle “n”, with respect to an analog signal which should be sampled inthe next cycle “n+1”, the corresponding voltage follower amplifier VFA(n+1) may be brought into the active state. As a result, the cycle timeof the A/D converting process operation may be shortened only by theactivation time “t1” of the voltage follower amplifier shown in FIG. 16.

[0113] As previously described, in the A/D converting circuit of thisembodiment, the voltage follower amplifier VFA is brought into theactive state in the front half time period of the sampling time period,and also, the MOSFET of the analog switch is turned ON in the rear halftime period of the sampling time period. As a result, when the analogswitch is turned ON, the capacitor of the sample-and-hold circuit SH hasalready been charged, or discharged up to such a level approximated tothe level of the analog input signal by way of the voltage followeramplifier VFA. As a consequence, as previously explained in the summaryof the invention, the present invention can extremely reduce theadverse-influenced amount of the level of the next analog input signalwhen the MOSFET of the analog switch is turned ON, and thisadverse-influenced amount is caused by the sampling electric chargeswhich have been stored in the stray capacitor (namely, samplingcapacitor in this embodiment) in the preceding cycle.

[0114] Accordingly, it is possible to reduce the capacitance values ofthe externally-connectable capacitors Ci to Cin which are required to beconnected to the analog input terminals AIN1 to AINn so as to reduce theerror of the analog input voltage which is sampled. As a result, evenwhen the analog signal source owns the relative large internalimpedance, since the input propagation delay time of the signal isshorter than that of the conventional circuit, even if the sampling timeperiod, namely the time period of the A/D converting operation is notmade long, then both the high-precision sampling result and also thehigh-precision A/D converting result can be obtained. Alternatively,such an analog signal source (sensor and high impedance noise preventingcircuit) may be employed, and the internal impedance of this analogsignal source is higher than that of the conventional A/D convertingcircuit.

[0115]FIG. 18 graphically shows a relationship between an internalimpedance Rin of an analog signal source and an error δ(LSB) containedin an analog input voltage Vin which is sampled by the sample-and-holdcircuit SH in such a case that while the capacitance value of theexternally-connectable capacitor Ci connected to the analog inputterminals AIN1 to AINn is selected to be 0.1 μF, the control operationis carried out at the timing as indicated in FIG. 17 in the A/Dconverting circuit of this embodiment. The error δ(LSB) may be expressedby the below-mentioned formula, assuming now that the resolution of theA/D converting circuit is 10 bits, the reference voltage is “Vref”, anda voltage which is actually stored in the sampling capacitor Cs is“Vsmp”:

δ(LSB)=(Vin−Vsmp)/(Vref/1024)

[0116] A reduction degree of the error in the following assumption isrepresented in FIG. 18. This assumption is defined by that the inputrange of the analog input voltage Vin is selected to be 0V to 5V; thesampling time is selected to be 3.2 μS; the A/D converting time isselected to be 15 μS; the stray capacitance of the connection nodebetween the analog multiplexer circuit and the sample-and-hold circuitis 50 pF; and also the potential difference between the analog inputterminal and the output-sided node after the charging operation by thevoltage follower amplifier is selected to be 100 mV. In the A/Dconverting circuit of this embodiment, the potential difference can beimproved by 100 mV/5V, as compared with that of the conventional A/Dconverting circuit (FIG. 21), and the error can be reduced byapproximately {fraction (1/50)} in direct proportional to thisimprovement in the potential difference. Also, if the error “δ” of theanalog input voltage Vin is allowable as the same error of approximately+0.5(LSB) and −0.5(LSB) in the prior art, then such anexternally-connectable capacitor having a capacitance of approximately0.01 μF may be connected to the analog input terminal. As a consequence,in the case that the internal impedances of the analog signal sourcesare identical to each other, since the present invention is applied tothe A/D converting circuit, the input propagation delay time may bereduced by approximately {fraction (1/10)}. As a result, even when thesampling time period is not made long, namely the time period of the A/Dconverting operation need not be prolonged, the high-precision samplingresult and the high-precision A/D-converting result may be obtained.

[0117] Furthermore, in the case that signals derived from a plurality ofanalog signal sources such as sensors are A/D-converted by a single A/Dconverting circuit in a periodic manner, in such a system which employsthe conventional multiplexer and the conventional sample-and-holdcircuit, when an analog switch is turned ON in a periodic manner, theanalog switch and an internal stray capacitor in combination operatesimilarly to a switched capacitor circuit. In this system, when theanalog switch is turned ON, a charge voltage of a sampling capacitor isliable to leak to the side of the input terminal. In other words, theoperation condition of this system is equal to such a condition that theinput terminal and the output side are equivalently connected via aresistor to each other. As a result, the following phenomenon occurs.That is, the voltage drop which is directly proportional to the electronchange stored in the internal stray capacitor is produced in theimpedance component of the analog single source, so that the level ofthe analog input signal is shifted form the actual voltage value of theanalog signal source. Contrary to that, when the present invention isapplied to such a system, since the analog switch is turned ON after thepotential difference between the input terminal and the output sidethereof has been decreased, in such a case that a plurality of signalsare sampled in the periodic manner by employing the multiplexer, a shiftamount (deviation amount) of an analog input signal from the actualvoltage value can be reduced.

[0118] Next, referring now to FIG. 19, a description will be made of anexample of a suitably-designed system LSI with employment of the A/Dconverting circuit according to the above-explained embodiment. FIG. 19represents a structural block diagram of a single-chip microcomputer asone example of such a system LSI containing the A/D converting circuit.Although no specific limitation is made in this example, the respectivecircuit blocks shown in FIG. 19 are manufactured on a singlesemiconductor chip such as single crystal silicon in accordance with theknown manufacturing technique for semiconductor integrated circuits.

[0119] As indicated in FIG. 19, the microcomputer of this embodiment isequipped with a program control type central processing unit CPU, aread-only memory ROM, a random access memory RAM, a calculation unitMULT, and a DMA controller DMAC. The read-only memory ROM storesthereinto a program executed by the CPU, and fixed data which isrequired for control operation. The random access memory RAM provides awork area of the CPU, and further, temporarily stores thereinto suchdata which is obtained in connection with an execution of a program. Thecalculation unit MULT performs a calculation process operation such as afloating decimal point calculation and the multiplication, instead ofcalculations executed by the CPU. Instead of this CPU, the DMAcontroller DMAC executes a DMA (direct memory access) type data transferoperation between the internally-provided RAM and an externally-providedstorage apparatus such as a hard disk apparatus. These various circuitsare connected via a CPU address bus IAB and a CPU data bus IDB to eachother.

[0120] In the microcomputer of this embodiment, both a peripheraladdress bus PAB and a peripheral data bus PDB are provided irrespectiveof the above-explained CPU buses IAB and IDB. To the peripheral addressbus PAB and the peripheral data bus PDB, an interrupt controller INTC; ause brake controller UBC; a peripheral module; and an input/output portare coupled. The interrupt controller INTC issues an interrupt commandwith respect to the CPU based upon an issue of a preselected interruptrequest. The user brake controller UBC requests the CPU to stop theexecution of the program at a brake point which is designated by a userduring emulation. The peripheral module is realized by, for example, aserial communication interface SCI which performs a serial communicationamong an analog-to-digital converting circuit ADC, a watch-dog timerused to detect an abnormal condition of hardware, and a timer circuitTIM external apparatus used to manage time. The input/output port PORTinputs/outputs a signal between this input/output port and an externalapparatus. It should also be noted that both the CPU buses IAB and IDBto which the CPU is connected are also connected to this input/outputport PORT, and thus, either the CPU or the DMAC may directlytransmit/receive data with respect to the external apparatus.

[0121] Also, a bus state controller BSC is provided between theabove-explained CPU buses IAB/IDB and the above-described peripheralbuses PAB/PDB. This bus state controller BSC adjusts timing of signalsappeared on these two buses so as to bridge signals between the CPU andthe peripheral module. Although no specific limitation is made, theabove-explained ROM is constituted by employing a flash memory in thisembodiment. This flash memory is capable of erasing data in a batch modein the unit of a predetermined data block. In addition, a clockgenerating circuit CPG is provided in this microcomputer, and generatesa clock which is required for an internal operation of thismicrocomputer. A crystal oscillator XTAL having a preselected naturalfrequency is coupled via an external terminal to this clock generatingcircuit CPG. Then, as the above-explained A/D converting circuit ADC,such an A/D converting circuit as described in the embodiment of FIG. 15is employed. In the automobile control system, as shown in FIG. 2, thedetection signals derived from the various sorts of sensors are enteredinto this A/D converting circuit ADC so as to be A/D-converted into thevarious digital sensor signals. Then, these digital sensor signals areprocessed by the CPU and the like.

[0122] While the present invention has been described with reference tothe embodiments exemplified by way of the concrete examples, the presentinvention is not limited only to the above-explained embodiments, butmay be modified, changed, or substituted without departing from thescope and spirit of the present invention. For instance, in theabove-described embodiment, as one example of the A/D convertingcircuit, such an example using the sequentially comparing type A/Dconverting circuit has been described. The present invention is notlimited to such a sequentially comparing type A/D converting circuit,but may employ other types of A/D converting circuits, for sample, anelectron charge redistributing type A/D converting circuit. Also, as thetransfer gate which constitutes the analog switch, the present inventionis not limited only to a COMS transfer gate, but may employ a singlechannel type MOSFET.

[0123] Also, while the present invention has been mainly described withrespect to the single-chip microcomputer containing the A/D convertingcircuit which is used in the automobile control system corresponding tothe utilization field, the present invention is not limited thereto.Alternatively, generally speaking, the present invention may be widelyutilized in such an A/D converting circuit employed in a system in whicha plurality of analog signals are A/D-converted by a single set of A/Dconverting circuit in a time divisional manner.

[0124] Some of the advantages which can be achieved by the typicalinventive ideas among the disclosed present invention will now be simplyexplained as follows:

[0125] That is, in both the analog switch circuit and the multiplexercircuit, to which the present invention has been applied, the electroncharges which have been stored in the stray capacitance before theswitch is operated do not give adverse influence to the level of thenext analog input voltage which is entered after the switch has beenoperated, but the analog signal can be transferred in high precision.

[0126] Also, in accordance with the A/D converting circuit to which thepresent invention has been described, even when the analog signal sourcehaving the high impedance is employed and also the filter circuitarrangement capable of withstanding the externally-supplied noise isused, the desirable A/D conversion precision can be obtained. Also, inaccordance with the system in which a plurality of analog signals areA/D-converted by a single A/D converting circuit in the time divisionalmanner, while using the multiplexer, the sampling period can beshortened without sacrificing the A/D conversion precision, and also,the A/D converting operation with the better response characteristic canbe carried out.

What is claimed is:
 1. An analog switch circuit comprising: a transfergate constructed of an insulating gate type transistor connected betweenan analog input terminal and an analog output terminal; and a voltagefollower circuit which is connected in parallel to said transfer gatebetween said analog input terminal and said analog output terminal;wherein: said voltage follower circuit is brought into an active stateimmediately before said transfer gate is conducted.
 2. An analog switchcircuit as claimed in claim 1 wherein: said transfer gate corresponds toa CMOS transfer gate constituted by both a p-channel MOSFET and ann-channel MOSFET which are connected in parallel to each other betweensaid analog input terminal and said analog output terminal.
 3. An analogswitch circuit as claimed in claim 1 wherein: said transfer gate isarranged by that two pieces of p-channel MOSFETs connected in a seriesconnection mode and two pieces of n-channel MOSFETs connected in aseries connection mode are parallel-connected between said analog inputterminal and said analog output terminal.
 4. An analog multiplexercircuit comprising: a plurality of analog input terminals; and onecommon analog output terminal; wherein: the analog switch circuitrecited in claim 1 is connected between each one of said plurality ofanalog input terminals and said common analog output terminal, and anyone of said analog switch circuits is selectively brought into aconductive state in response to a control signal.
 5. An A/D convertercomprising: the analog switch circuit recited in claim 1, which isconnected to an analog input terminal; a sampling capacitor which isconnected to an analog output terminal of said analog switch circuit;and an analog-to-digital converting circuit for converting an analogsignal acquired in said sampling capacitor into a digital signalcorresponding thereto.
 6. An analog switch circuit wherein: both atransfer gate constituted by that two pieces of p-channel MOSFETsconnected in a series connection mode and two pieces of n-channelMOSFETs connected in a series connection mode are parallel-connectedbetween an analog input terminal and an analog output terminal, and alsoa voltage follower circuit connected in parallel to said transfer gatebetween said analog input terminal and said analog output terminal aremanufactured on a single semiconductor substrate; both said twop-channel MOSFETs connected in the series connection mode and said twon-channel MOSFETs connected in the series connection mode are formed ontwo semiconductor regions respectively, said two semiconductor regionsbeing formed under such a condition that said two semiconductor regionsare separated on a major surface of said semiconductor substrate; andsaid transfer gate is conducted after said voltage follower circuit hasbeen brought into an active state.
 7. An analog multiplexer circuitcomprising: a plurality of analog input terminals; and one common analogoutput terminals; wherein: the analog switch circuit recited in claim 6is connected between each one of said plurality of analog inputterminals and said common analog output terminal, and any one of saidanalog switch circuits is selectively brought into a conductive state inresponse to a control signal.
 8. An analog signal processing systemcomprising: an analog multiplexer circuit constituted by a plurality ofanalog signal sources, a plurality of analog input terminals to whichsaid plural analog signal sources are connected, and a plurality ofanalog switch circuits which are parallel-connected between said pluralanalog input terminals and a common analog output terminal; asample-and-hold circuit constituted by an analog switch and a samplingcapacitor, said analog switch transferring an analog signal selected bysaid analog multiplexer circuit; and an analog-to-digital convertingcircuit for converting an analog signal held by said sample-and-holdcircuit into a digital signal corresponding thereto; wherein: saidanalog switch circuit is arranged by plural transfer gates, each ofwhich is made of an insulating gate type transistor, and also pluralvoltage follower circuits which are connected in parallel to saidrespective transfer gates; and each of said plural analog switchcircuits is arranged in such a manner that after said voltage followercircuit of said analog switch circuit has been brought into an activestate, said transfer gate is conducted.
 9. An analog signal processingsystem as claimed in claim 8 wherein: said transfer gate is constitutedby both a p-channel MOSFET and an n-channel MOSFET which are connectedin parallel to each other between said analog input terminal and saidanalog output terminal.
 10. An analog signal processing system asclaimed in claim 9 wherein: said transfer gate is arranged by that twop-channel MOSFETs connected in a series connection mode and two piecesof n-channel MOSFETs connected in a series connection mode areparallel-connected between said analog input terminal and said analogoutput terminal.